The present invention relates to cleaning processes of mixed metals and, more particularly, to an improved method for removing metals which accumulate in citric acid cleaning solutions thereby permitting the subsequent discharge or recycling of the liquid solution.
Citric acid cleaning processes are used in a variety of applications. For example, such cleaning processes are typically employed in large degreasing and cleaning shops. Due to the corrosive effects of the citric acid, the cleaning solutions often contain large amounts of the metals being treated. In particular, soft metals such as iron and aluminum will collect in the solution.
Although citric acid cleaning techniques have been very successful over the years from a cleaning standpoint, the treatment of the cleaning solutions prior to disposal have proven to be quite problematic, primarily due to the difficulty in removing metals from the cleaning solution. For example, even after the metals in solution are rendered insoluble, some amount of the metals remains in a colloidal suspension. These suspended metals are exceedingly difficult to filter out of the solution. Additionally, the suspended metals tend to bind with the filter medium.
Methods for the removal of the metals from the citric acid cleaning solutions typically involve a one-step pH process. In this process the pH of the cleaning solution is adjusted to a pH of between 7.5 and 8.5 through the addition of a caustic such as NaOH. The solution is then stabilized and filtered. The efficiency of this approach is primarily driven by the composition of the cleaning solution, both in terms of the concentrations of the metals and the molarity of the citric acid.
What is needed in the art is a new method of removing metals from citric acid cleaning solutions that is more efficient than the prior techniques. The present invention provides such a process.
The process of the present invention repeatedly manipulates the pH of a cleaning solution, utilizing the corrosion products of iron and aluminum as flocculating agents to optimize the removal of other metals from the solution. Two separate precipitation/flocculation/filtration processes are performed at different pH levels.
The first step of the metals removal process is to adjust and stabilize the pH of the solution to approximately 11.5, preferably through the addition of sodium hydroxide. At this level of alkalinity, aluminum hydroxide is soluble while most other metals contained within the solution are insoluble. Insoluble iron hydroxide is formed, acting as a flocculant to optimize phase separation. After phase separation, the solution is filtered and the filtrate is collected. The pH of the filtrate is then reduced to approximately 8.0, preferably through the addition of nitric acid. At this pH aluminum hydroxide becomes insoluble and acts as a second flocculant. A second filtration step is then performed, the remaining filtrate having a greatly reduced concentration of metals.
In at least one embodiment of the invention, after the two-step pH adjustment/filtration process, an additional step is performed in which the filtrate is subjected to an ion exchange resin. Preferably a cationic exchange resin such as Purolite S-940 or S-950 is used. More preferably Purolite S-950 converted to a H+ form is used.
In at least one other embodiment of the invention, the cleaning solution is initially tested to determine the citric acid concentration. If the citric acid concentration is determined to be greater than or equal to 0.01 Molar, the two-step process of the invention is used. Alternately, if the citric acid concentration is determined to be less than 0.01 Molar, a one-step process is used in which the pH of the solution is adjusted and stabilized to between 7.5 and 8.5 and then the solution is filtered.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.